A compact, low wattage metal halide lamp of acceptable life should have a wall thickness sufficient to withstand the heat and pressure of the enclosed lamp fill during lamp operation. Wall thicknesses for compact metal halide lamps are usually in excess of about 1.25 millimeters (0.050 inch). There are two ways to make a lamp capsule with this wall thickness. One method is to start with, relatively thin walled tubing and gather the tube material in the cavity region to increase the wall thickness. Gathering the tube material results in end seals adjacent the cavity with low thermal masses, but unfortunately frequently results in irregular surfaces. An alternative is to mold or press thick walled tubing. However, thick walled tubing results in massive end seals.
Both methods introduce difficulties. The problem with gathering material is in obtaining a uniform cavity with a consistent wall thickness. The irregularities in wall thickness make thermal conduction and light distribution inconsistent. The problem with thick wall tubing is reduced lamp performance due to energy loss, and altered thermal patterns. The heavy end seals act as heat sinks which lower the cold point of the lamp envelope. An ideal discharge cavity should possess a uniform wall temperature distribution with no cold spot. The optimum temperature should be extremely high, but below the critical temperature at which chemical reactions with the envelope material, commonly quartz, become a problem. Unfortunately, heavy end seals cause thermal nonuniformity. It is probable that the principle heat loss mechanisms in the seals are thermal radiation and convection. The heavy seals rob performance in several ways. The high heat loss to the seal ends results in a direct power loss, and cold spots in the seal ends of the cavity. The seal ends also act as heat sinks during warm-up which increases the warm-up time. An oxidation problem may also occur during lamp operation. The large cross-sectional area in the end seal causes the molybdenum foil to reach excessively high temperatures. When the exposed molybdenum end runs too hot, rapid oxidation occurs, resulting in seal failure. There is then a need for an arc discharge lamp with regular capsule walls and a low mass seal.
Examples of the prior art are shown in U.S. Pat. Nos. 3,685,880; 3,689,799; 3,989,970; and 4,612,475. Sobieski U.S. Pat. No. 3,685,880 shows a method of making an arc discharge lamp capsule. The drawing shows a prolate capsule with thinner walls adjacent the electrodes. Senft U.S. Pat. No. 3,689,799 concerns a method of dosing an arc discharge capsule, but shows an elongated capsule with thinner walls adjacent the electrode roots. Downing U.S. Pat. No. 3,989,970 shows a double ended arc discharge capsule with slightly thinner walls adjacent the electrodes, but otherwise concerns radiator fins coupled to the press seals. Downing U.S. Pat. No. 4,612,475 shows a double ended arc discharge capsule with slightly thinner walls adjacent the electrodes, but otherwise concerns the lamp fill composition.